1. Field of the Invention
This invention relates to the cleaning of plastics, especially for purposes of recycling. More specifically, this invention comprises a method by which a liquid surfactant and a liquid caustic are separately introduced into the cleaning phase of a recycling system so as to increase the cleaning efficiency while reducing the presence of foam.
2. Related Art
Plastic recycling is a relatively new industry which is currently experiencing tremendous growth. Recent concerns about the environmental impact of plastic use and disposal, as well as recent legislation, have led to a widespread demand for recycling used plastic products into new, usable consumer goods. Plastics are a petroleum product, and concerns about oil conservation have also led to a demand to recycle post-consumer plastic rather than meeting demand with new plastic products.
Since the recycling business is relatively new, there are few standard methods for recycling plastics, and improved means for accomplishing any recycling step are constantly sought. While there are few industry standards, a typical recycling business collects used plastic products, and then sorts them according to the type and color of the plastic. The sorted plastics are then shredded by some means into small plastic chips or flakes. These chips must then be cleaned to remove any dirt, oil, food particles, labels, glue, or other contaminants which may be present. The clean chips are then usually pelletized, or somehow organized in a usable form for a plastic manufacturer.
The chips are usually cleaned by a continuous flow process which introduces an aqueous cleaning solution and agitates and scrubs the plastic chips to remove contaminants. Some recycling businesses employ a batch process for cleaning. The cleaning process has been generally operated at a temperature of about 190.degree. F. After cleaning, the chips have been typically moved on to a drying process, while the cleaning solution has been filtered, recycled, and renewed for further use. The renewal process has usually involved filtering the cleaning solution to remove glue and paper, followed by a blowdown of about 10% of the old solution, and then adding some fresh solution and recycling the solution to the cleaning phase.
Typically, the cleaning solution has contained a surfactant with sufficient cleaning properties to clean the plastic chips, and sufficient caustic compound to maintain an alkalinity adequate to remove labels and glue. Few commercial cleaning products are offered specifically for cleaning plastics in a recycling process. A general discussion of the effectiveness of several commercially available detergents for cleaning polyethylene chips is provided by Dr. Sidney Ranking, Ph.D. in "Technical Report #14: Process Simplification-Washing Technology", published by Rutgers University Center for Plastics Recycling Research, 1988. That article examines and compares several detergents, some of them household dishwashing detergents, which the solid either as solids or highly concentrated liquids and must be dissolved in water to create a cleaning solution.
These commercially available detergents generally comprise both a surfactant and a caustic in constant weight percentages. Such detergents may be a hindrance if a mixture of plastics from various sources is to be cleaned by the same system. The only way to increase the effectiveness of a cleaning solution prepared with these detergents has been to add more detergent to the solution. Unfortunately, this practice can cause excessive foaming, which may not be adequately rectified by using anti-foam products; moreover, the cleaning solution may still be inadequate to clean a particular plastic.
There can also be a large unnecessary expense associated with the use of such detergents. If more caustic is needed in the cleaning solution to help remove glue and labels from the plastic, there is no way to add more caustic without also adding more surfactant to the solution. Conversely, if only a small amount of caustic is needed, there is typically no way to reduce the amount of caustic in the cleaning solution without changing detergents. Significant expense is involved in using the wrong quantities of an inadequate detergent, when a lesser amount of a different cleaning solution would satisfactorily clean the plastic. Thus there is a continuing need for a method for cleaning plastic which can be adequately modified to meet the needs of a particular recycling system, and for monitoring the cleaning solution to help ensure that a proper amount of cleaner is constantly present.